An increasingly popular method of producing mechanical pulp for paper, paper board, and the like is refiner technology. In conventional refiners (whether using disks, or cylindrical or conical refiner elements), the lignin in the cellulosic fibrous material (e.g. wood chips, or the like) that provides the raw material for pulping, is softened by compression and decompression and by the friction of wood-to-wood and metal-to-wood provided by the relatively rotating refiner elements. A tension field is created utilizing the refiner bars for compression, shear forces, and decompression. A tension field exists between the bars. Most of the . refiner energy applied is used to refine the fibers and improve the flexibility and bonding ability. Fiber rolling motion is desirable, but much fiber cutting action occurs.
According to the present invention, by increasing and tailoring the shear forces applied during mechanical refining of paper pulp, the fiber flexibility and paper strength properties are improved. According to the present invention, more fiber rolling motion is applied to the raw material, and less fiber cutting. Thus the intensity of the energy supply increases. The teachings of the invention can be applied to each kind of refiner element. Also, the technology is applicable to low frequency refining, such as disclosed in U.S. Pat. 4,754,935, the disclosure of which is hereby incorporated by reference herein.
Conventional refiners typically have parallel grooved bottoms between the bars except where the spacing between the refiners (grooved width) is very small. In the latter situation, the grooved bottom is typically curved, having a radius of curvature. This results in all of the compression, shear forces, and decompression taking place when the refiner bars are aligned with each other on the relatively rotating refiner elements.
According to the present invention, an apparatus and method are provided for producing a mechanical pulp having increased fiber flexibility, while the content of long fibers thereof is maintained at a high proportion. This is accomplished, according to the invention, by providing grooves of a slightly greater width than is conventional between the bars of the refiners, and providing a sloping bottom of the grooves in order to provide additional shearing forces. Typically, a refiner element according to the invention has a groove width of about 10-50 mm, and the grooved bottom slopes downwardly from adjacent one bar to adjacent the next bar at an angle of about 1.degree.-30.degree. (preferably about 5.degree.-20.degree.) to a straight line between the bars. The relatively rotatable refiner elements according to the invention have comparable configurations, and desirably the widths of the grooves on the elements are the same, integer multiple of the number of bars of one element than of the other element.
Utilizing the refiner elements according to the invention and effecting relative rotation therebetween, a method of refining--with the coined name of "Sequential Refining"--is possible. According to the method of the invention, refining of a slurry of cellulosic fibrous material into paper pulp is effected by causing relative rotational movement of the refiner elements with respect to each other to continuously and successively provide a moving tension field, with successive compressions before impacts, and expansions, to achieve increased fiber flexibility, more fiber rolling motion and less fiber cutting than conventional refining.
During the practice of the invention, the consistency of the slurry is always between about 30-55% solids. In addition to producing RMP, the invention can be practiced to produce thermomechanical pulp (TMP), chemimechanical pulp (CMP), and chemithermomechanical pulp (CTMP), or other high-yield or mechanical pulps by related methods of production.
The invention is capable of supplying increased shear forces by utilizing a moving tension field, with successive compressions before impact, and expansions, achieving increased fiber flexibility, more fiber rolling motion, and less fiber cutting, than conventional refining. Increased paper strength properties, and higher refiner capacity due to higher intensity of energy supply resulting in lower demand of specific energy, ensue. The invention teachings are applicable to all conventional types of refiner segments, including disks (single disk or double disk), cylinders, or conical refiner elements.
The refiner of the invention may also be utilized for mixing chemicals into kraft (chemical) pulp. By passing kraft pulp at high consistency (e.g. 30-55%) and chemicals through a refiner, the moving tension field produced according to the invention achieves fiber rolling and kneading action, the chemical penetrating the fibers. Through the action of the inherent great number of pulsations, the liquid inside and outside the fibers is equalized with treatment chemical (e.g. bleaching liquid). Thus treatment chemical can be evenly distributed in the pulp at high consistency and temperature. This results in reduced chemical consumption for a given treatment (e.g. bleach) level, and enormous savings in equipment space requirements.
It is the primary object of the present invention to provide for the production of mechanical pulp with increased fiber flexibility at a maintained high content of long fibers. This and other objects of the invention will be seen from an inspection of the detailed description of the invention, and from the appended claims.